Project 2 Summary Sensory perception depends on neocortical computations that contextually adjust signals from sensory organs with internal information (top-down modulation). Sensory information is first conveyed by the primary sensory thalamus to neocortical layers 4 and 5b/6 and is eventually relayed to the basal dendrites of pyramidal cells. In contrast, top-down information, which projects from many different sources, including cortical and subcortical regions, predominately targets upper ?associative? cortical layers. In the neocortex, GABAergic interneurons (INs) are highly diverse and have critical roles in sculpting the spatiotemporal aspects of circuit activity. Moreover, malfunction of neocortical GABAergic interneurons has been linked with diseases including epilepsy, schizophrenia, anxiety, and autism spectrum disorders. INs of the 5HT3aR family are the main IN population in the upper layers. The goal of Project 2 is to test the hypothesis that 5HT3aR cortical INs have important roles in context-dependent sensory processing and is focused on neocortical layer 1 (L1). This layer is the main target of top-down input. It contains no excitatory cells, but is populated by 5HT3aR INs. During the previous funding period of this PPG, we made significant advances in understanding the neuronal composition of L1 and generated experimental reagents to target these neurons. Based on these advances Project 2 will elucidate the L1 microcircuits mediating top-down modulation of sensory processing. In Aim 1 we will utilize paired whole-cell recordings in slices, monosynaptic rabies tracing, and functional optogenetic mapping to characterize the input and output connectivity of L1 IN subtypes in primary somatosensory cortex in order to elucidate the neuronal networks that process contextual information in L1. In Aim 2 will use in vivo recordings in awake mice and pharmacogenetic manipulations to investigate how L1 integrates primary sensory information with context dependent information conveyed by two distinct functionally and clinically important pathways that differentially impact the activity of pyramidal cells.